1. Field of the Invention
The present invention relates to a power supply apparatus. More particularly, the present invention relates to an operation-mode determining unit of a power supply apparatus and an operation-mode determining method thereof.
2. Description of Related Art
Power management circuits play an important role in current electronic products, for example, a DC Buck converter is commonly-used and an important power management technique. In various designs of DC converters, it is an important subject to successfully switch to a pulse-width modulation (PWM) mode and a pulse-frequency modulation (PFM) mode according to the magnitude of load current. FIG. 1 illustrates the relationship between the conversion efficiency and the load current in the PWM mode and the PFM mode. Referring to FIG. 1, for a light load power supply (i.e., low output current), the power supply apparatus must be designed to adopt the PFM mode to regulate the voltage. On the contrary, for a heavy load power supply (i.e., high output current), the power supply apparatus must be designed to adopt the PWM mode to regulate the voltage.
In order to allow a power supply apparatus operate in application environments of both light load and heavy load, the power supply apparatus must have both the PWM mode and the PFM mode and automatically select one of them according to the load state. For example, U.S. Pat. No. 6,433,525 of Intersil Corporation uses a circuit of detecting a zero inductor current to detect whether or not the polarity of the inductor current has been changed, and adopts a counter to delay the switching time sequence of the operation mode.
FIG. 2A is a block diagram of U.S. Pat. No. 6,433,525. FIG. 2B is a schematic view of waveforms of signals of FIG. 2A. As known from FIGS. 2A and 2B, the conventional art uses a circuit for detecting the zero inductor current in a mode control 50 to detect whether or not the direction of the inductor current IIND has been changed. When the inductor current IIND changes the direction, a comparator in the mode control 50 outputs a control signal to the counter in the mode control 50. When the signal output from the comparator is positive and the count of the counter reaches the predetermined number of times, the mode control 50 outputs a logic signal to perform switching from the PWM mode to the PFM mode. On the contrary, when the signal output from the comparator is negative and the count of the counter reaches the predetermined number of times, the output logic signal is changed to perform switching from the PFM mode to the PWM mode.
In the above technology, the counter is required to count the time since the load state starts to change, i.e., the counter is used to provide a delay switching mechanism. However, in the conventional art, as the judgment time of a hysteretic counter is required to determine whether or not to switch the operation mode, the response time of the power supply apparatus is impacted significantly.